Gas barrier films, each composed of a laminate of multiple layers including thin films of metal oxides, such as aluminum oxide, magnesium oxide, and silicon oxide, formed on a surface of a plastic substrate or a resin film, have been widely used for wrapping products that must block gases, such as water vapor and oxygen, more specifically for wrapping foods, industrial products, and pharmaceuticals to prevent alterations in their properties.
In addition to wrapping, gas barrier films have been widely studied for the application to sealants for electronic devices, such as flexible photovoltaic cells, organic electroluminescent (EL) elements, and liquid crystal display devices. Such sealants for electronic devices require excellent gas barrier properties equivalent to those of glass substrate material. Unfortunately, a gas barrier film that has sufficient capability has not yet been produced.
Known procedures for producing gas barrier films include gas phase processes, such as chemical deposition (plasma-assisted chemical vapor deposition (CVD)) and physical deposition (vacuum deposition and sputtering). In chemical deposition, organosilicone compounds, such as tetraethoxysilane (TEOS), are oxidized in oxygen plasma under reduced pressure while depositing an inorganic film on a substrate; while in physical deposition, metallic silicon is vaporized with a semiconductor laser to deposit an inorganic film on a substrate in the presence of oxygen.
Such gas phase processes have been preferably used for the formation of inorganic films composed of, for example, silicon oxide, silicon nitride, and silicon oxynitride. Thus, studies have been widely conducted on the compositions of inorganic films that have excellent gas barrier properties and the layer structure of such inorganic films. Unfortunately, the compositions and layer structures have not yet been specified that achieve excellent gas barrier properties.
It is significantly difficult to form an inorganic film without defects, such as pinholes and cracks, through the gas phase process described above. To prevent defects, for example, the film formation rate must be significantly reduced. Thus, a gas barrier film having properties that satisfy the requirements for a sealant for electronic devices at industrial levels in terms of productivity has not been produced. Studies have been conducted in the attempt to improve the gas barrier properties by simply increasing the thickness of an inorganic film produced through a gas phase process and forming a laminate of multiple inorganic films. Unfortunately, the gas barrier properties have not been improved due to the continuous growing of defects and/or increases in cracks.
In another study, an organic film has been disposed over an inorganic film formed through a gas phase process to form alternating layers of inorganic films and organic films. In this way, the total thickness of the inorganic films can be maintained while preventing the continuous growth of defects in the inorganic films and increasing the length of the permeation pathway of gases due to the non-uniform positions of the defects on the surfaces of the inorganic films. Such effect is known as a labyrinth effect and has been studied to improve the gas barrier properties. Under the present circumstances, the gas barrier properties are not satisfactory, and the production process is complicated. That is, the productivity compatible with the performance of the gas barrier film is significantly low. Thus, practical applications are deemed difficult due to costs.
A technical approach has been conducted on a gas barrier layer to provide a solution to the issues mentioned above. The gas barrier layer is formed by applying a solution of an inorganic precursor compound to a substrate and drying the substrate to form a film which is then modified, for example, by heat to improve the gas barrier properties. Specifically, a study has been conducted on the use of polysilazane as an inorganic precursor compound in order to achieve excellent gas barrier properties.
Polysilazane is a compound having a —(SiH2—NH)— basic structure, e.g., perhydropolysilazane. Heat treatment or humid heat treatment of polysilazane in an acidic atmosphere causes the polysilazane to chemically change to silicon oxynitride and then to silicon oxide. The direct replacement of nitrogen with oxygen by the action of oxygen and water vapor in the atmosphere causes a chemical change to silicon oxide with a relatively small volume reduction. Thus, it is known that a relatively dense film can be acquired with fewer defects due to a volume reduction. A relatively dense silicon oxynitride film can also be acquired through the control of the acidity of the atmosphere.
The formation of a dense silicon oxynitride or silicon oxide gas barrier layer through heat modification or humid heat modification of polysilazane requires a temperature of 450° C. or higher. Thus, a gas barrier layer cannot be formed on a flexible substrate (resin substrate), such as a plastic substrate.
A proposed solution to this problem is a method of forming a silicon oxynitride film or silicon oxide film by applying a polysilazane solution to form a film and modifying the film through irradiation with vacuum ultraviolet rays. Light (wavelength of 100 to 200 nm) referred to as vacuum ultraviolet rays (hereinafter referred to as “VUV” or “VUV light”), which has an energy greater than the interatomic bonding force of polysilazane, is used to promote oxidation by active oxygen or ozone while directly breaking the bonds between the atoms in polysilazane by only photons through a light quantum process. In this way, a silicon oxynitride film or silicon oxide film can be formed at a relatively low temperature.
Non-patent literature 1 discloses a method of producing a gas barrier film by irradiating polysilazane film with VUV light from an excimer lamp. In non-patent literature 1, the layer structure and production conditions of the gas barrier film are not studied in detail. Thus, the gas barrier properties of the resulting gas barrier film are far from the properties required for electronic devices.
Patent literature 1 discloses a method of producing a gas barrier film by irradiating polysilazane film containing a basic catalyst with VUV light and UV light. An embodiment thereof describes an example gas barrier film having three gas barrier layers formed by applying polysilazane to a resin substrate, drying the polysilazane layer, and irradiating the polysilazane with VUV light. However, the gas barrier properties of the resulting gas barrier film are far from the properties required for electronic devices. Patent literature 1 lists binder resins commonly used in the production of paint containing polysiloxane as a component other than a catalyst that can be added to a coating solution containing polysilazane. However, patent literature 1 does not suggest the improvement in the gas barrier properties through the addition of an appropriate amount of a compound having a specific structure other than catalysts.
Patent literature 2 discloses a gas barrier film including a laminate of two or more gas barrier layers, each layer being formed by applying a polysilazane film having a thickness of 250 nm or less on a resin substrate having a smooth surface (having a surface Ra value of less than 12 nm) and irradiating the polysilazane film with VUV light. A thin polysilazane film is provided in the attempt to prevent the generation of cracks during VUV light irradiation and improve the gas barrier properties through increases in the number of layers and the thickness of the film. Even if a laminate is formed of thin layers, cracks are generated at a total thickness of the gas barrier layers exceeding a certain value. Thus, the gas barrier properties required for electronic devices cannot be achieved. The productivity, including yield, of this approach is not high because a multi-layer structure is a prerequisite and the resin substrate having a smooth surface is prone to damage during shipping.
Patent literature 3 discloses a gas barrier film including an inorganic-organic composite copolymer film containing heat-curable silicone, which is a mixture of polysilazane and polysiloxane polymers. Although the inorganic-organic composite copolymer film does not substantially have gas barrier properties, it provides improved adhesiveness with an inorganic oxide layer having gas barrier properties disposed on the copolymer film and prevents cracks from forming in the inorganic oxide layer due to stress release. Patent literature 3 does not suggest the improvement in the gas barrier properties through the addition of an appropriate amount of compound having a specific structure to polysilazane and the irradiation with VUV light.
Gas barrier films have been in need that have excellent gas barrier properties required for electronic devices and excellent productivity.